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Contracts and Agreements (Brokerage Commissions) (Details) (USD $) | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Long Duration Contracts Assumptions By Product And Guarantee [Line Items] |  |
Brokerage commissions | $ 6,437 us-gaap_FloorBrokerageExchangeAndClearanceFees | $ 1,400 us-gaap_FloorBrokerageExchangeAndClearanceFees
Brokerage commissions as a percentage of average daily net assets | 0.04%usl_PercentageOfAverageDailyNetAssets | 0.01%usl_PercentageOfAverageDailyNetAssets
Rebalancing Investments Transaction [Member] |  |
Long Duration Contracts Assumptions By Product And Guarantee [Line Items] |  |
Brokerage commissions | 4,323 us-gaap_FloorBrokerageExchangeAndClearanceFees / us-gaap_NatureOfExpenseAxis = usl_RebalancingInvestmentsTransactionMember | 999 us-gaap_FloorBrokerageExchangeAndClearanceFees / us-gaap_NatureOfExpenseAxis = usl_RebalancingInvestmentsTransactionMember
Brokerage commissions as a percentage of average daily net assets | 67.16%usl_PercentageOfAverageDailyNetAssets / us-gaap_NatureOfExpenseAxis = usl_RebalancingInvestmentsTransactionMember | 71.36%usl_PercentageOfAverageDailyNetAssets / us-gaap_NatureOfExpenseAxis = usl_RebalancingInvestmentsTransactionMember
Trades Necessitated By Creation And Redemption Activity [Member] |  |
Long Duration Contracts Assumptions By Product And Guarantee [Line Items] |  |
Brokerage commissions | $ 2,114 us-gaap_FloorBrokerageExchangeAndClearanceFees / us-gaap_NatureOfExpenseAxis = usl_TradesNecessitatedByCreationAndRedemptionActivityMember | $ 401 us-gaap_FloorBrokerageExchangeAndClearanceFees / us-gaap_NatureOfExpenseAxis = usl_TradesNecessitatedByCreationAndRedemptionActivityMember
Brokerage commissions as a percentage of average daily net assets | 32.84%usl_PercentageOfAverageDailyNetAssets / us-gaap_NatureOfExpenseAxis = usl_TradesNecessitatedByCreationAndRedemptionActivityMember | 28.64%usl_PercentageOfAverageDailyNetAssets / us-gaap_NatureOfExpenseAxis = usl_TradesNecessitatedByCreationAndRedemptionActivityMember

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- Definition The amount of expense during the period for floor brokerage fees paid to other broker-dealers to execute trades on their behalf, stock exchange fees, order flow fees, and clearance fees. + References No definition available. + Details
Name: | us-gaap_FloorBrokerageExchangeAndClearanceFees
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
Name: | us-gaap_LongDurationContractsAssumptionsByProductAndGuaranteeLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Percentage of Average Daily Net Assets + References No definition available. + Details
Name: | usl_PercentageOfAverageDailyNetAssets
Namespace Prefix: | usl_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Details
Name: | us-gaap_NatureOfExpenseAxis=usl_RebalancingInvestmentsTransactionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_NatureOfExpenseAxis=usl_TradesNecessitatedByCreationAndRedemptionActivityMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |